Video cameras and digital still cameras have employed auto white balancing adjustment with the aim of reproducing a white subject so that it looks white. There are many examples of automated prior art in this field.
U.S. Pat. No. 5,801,773 (Ikeda) teaches performing white balance using averages of groups of pixels (paxels) that fall within a broadened color temperature axis region in a color difference space and are confined to spatial regions of the image corresponding to user mode settings, e.g., landscape, portrait, etc. The shape of the color temperature axis region is also modified (usually reduced) by the user mode settings.
U.S. Pat. No. 6,243,133 (Spaulding, et al.) teaches producing a paxelized version of the image to be scene balanced, transforming said paxelized image into a gamut-preserving version of CIE XYZ space, computing the equivalent scene balance correction triplet, and back-transforming the triplet into the original camera RGB space so that it can be applied directly to the full-resolution original image to produce a scene-balanced image in a gamut-preserving CIE XYZ space suitable for subsequent output rendering.
U.S. Pat. No. 6,791,606 (Miyano) teaches producing a paxelized image and converting it into a color difference space. The distance of each paxel in the color difference space from predefined light source regions is then computed and treated as a reliability value. (The farther the paxel is from a given illuminant, the less reliable is the conclusion that that paxelized region is illuminated by said illuminant.) The associated white balance corrections for each illuminant are blended using the reliability values to produce a global white balance correction.
U.S. Patent Application Publication No. 2002/0122120 (Hsieh) teaches segmenting the image into a number of regions and computing average color difference coordinates for each region. For regions with moderate values of luminance, the average color difference values are plotted in color difference space and each quadrant of the plot examined. Only points landing in the quadrant with the majority of plotted points are used for computing the global white balance correction.
A significant problem with existing white balance correction methods is that they do not correctly balance a scene that is exposed with two different illuminants (e.g., flash and tungsten).